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Synonyms include formonitrile. Aqueous solutions are referred to as hydrocyanic acid and prussic acid.

Persons whose clothing or skin is contaminated with cyanide-containing solutions can secondarily contaminate response personnel by direct contact or through off-gassing vapor.

Hydrogen cyanide is a colorless or pale-blue liquid at room temperature. It is very volatile, readily producing flammable and toxic concentrations at room temperature. Hydrogen cyanide gas mixes well with air, and explosive mixtures are easily formed.

Hydrogen cyanide has a distinctive bitter almond odor, but some individuals cannot detect it and consequently, it may not provide adequate warning of hazardous concentrations.

Hydrogen cyanide is absorbed well by inhalation and can produce death within minutes. Substantial absorption can occur through intact skin if vapor concentration is high or with direct contact with solutions, especially at high ambient temperatures and relative humidity. Exposure by any route may cause systemic effects.

General Information

Description

At temperatures below 78 °F, hydrogen cyanide is a colorless or pale-blue liquid (hydrocyanic acid); at higher temperatures, it is a colorless gas. Hydrogen cyanide is very volatile, producing potentially lethal concentrations at room temperature. The vapor is flammable and potentially explosive. Hydrogen cyanide has a faint, bitter almond odor and a bitter, burning taste. It is soluble in water and is often used as a 96% aqueous solution (NIOSH 2005).

Routes of Exposure

Inhalation

Hydrogen cyanide is readily absorbed from the lungs; symptoms of poisoning begin within seconds to minutes. The odor of hydrogen cyanide is detectable at 2-10 ppm (OSHA PEL = 10 ppm), but does not provide adequate warning of hazardous concentrations. Perception of the odor is a genetic trait (20-40% of the general population cannot detect hydrogen cyanide); also, rapid olfactory fatigue can occur. Hydrogen cyanide is lighter than air (HSDB 2007).

Children exposed to the same levels of hydrogen cyanide as adults may receive larger doses because they have greater lung surface area:body weight ratios and increased minute volumes:weight ratios.

Skin/Eye Contact

Exposure to hydrogen cyanide can cause skin and eye irritation. More importantly, skin or eye absorption is rapid and contributes to systemic poisoning. After skin exposure, onset of symptoms may be immediate or delayed for 30-60 minutes. Most cases of toxicity from dermal exposure have been from industrial accidents involving partial immersion in liquid cyanide or cyanide solutions or from contact with molten cyanide salts, resulting in large surface-area burns (ATSDR 2006).

Children are more vulnerable to toxicants absorbed through the skin because of their relatively larger surface area:body weight ratio.

Ingestion

Ingestion of hydrogen cyanide solutions or cyanide salts can be rapidly fatal (ATSDR 2006). Treatment of ingested cyanide salts is similar to treatment of oral hydrogen cyanide poisoning because cyanide salts form hydrogen cyanide in acidic conditions.

Sources/Uses

Hydrogen cyanide is manufactured by oxidation of ammonia- methane mixtures under controlled conditions and by the catalytic decomposition of formamide. It may be generated by treating cyanide salts with acid, and it is a combustion by-product of nitrogen-containing materials such as wool, silk, and plastics. It is also produced by enzymatic hydrolysis of nitriles and related chemicals. Hydrogen cyanide gas is a by-product of coke-oven and blast-furnace operations (ATSDR 2006; Hartung 1994).

Hydrogen cyanide is used in fumigating; electroplating; mining; and in producing synthetic fibers, plastics, dyes, and pesticides. It also is used as an intermediate in chemical syntheses (Hartung 1994). Hydrogen cyanide may also be found in cigarette smoke (ATSDR 2006; HSDB 2007).

Health Effects

Hydrogen cyanide is highly toxic by all routes of exposure and may cause abrupt onset of profound CNS, cardiovascular, and respiratory effects, leading to death within minutes.

Exposure to lower concentrations of hydrogen cyanide may produce eye irritation, headache, confusion, nausea, and vomiting followed in some cases by coma and death.

Hydrogen cyanide acts as a cellular asphyxiant. By binding to mitochondrial cytochrome oxidase, it prevents the utilization of oxygen in cellular metabolism. The CNS and myocardium are particularly sensitive to the toxic effects of cyanide.

Acute Exposure

In humans, cyanide combines with the ferric ion in mitochondrial cytochrome oxidase, preventing electron transport in the cytochrome system and bringing oxidative phosphorylation and ATP production to a halt. The inhibition of oxidative metabolism puts increased demands on anaerobic glycolysis, which results in lactic acid production and may produce severe acid-base imbalance. The CNS is particularly sensitive to the toxic effects of cyanide, and exposure to hydrogen cyanide generally produces symptoms within a short period of time (ATSDR 2006).

Children do not always respond to chemicals in the same manner as adults. Different protocols for managing their care may be needed.

Cardiovascular

Abnormal heartbeat can occur in cases of severe poisoning. Slow heartbeat, intractable low blood pressure, and death may result. High blood pressure and a rapid heartbeat may be early, transient findings (ATSDR 2006).

Respiratory

After systemic poisoning begins, victims may complain of shortness of breath and chest tightness. Pulmonary findings may include rapid breathing and increased depth of respirations. As poisoning progresses, respirations become slow and gasping; a bluish skin color may or may not be present. Accumulation of fluid in the lungs may develop (ATSDR 2006).

Children may be more vulnerable to gas exposure because of relatively increased minute ventilation per kg and failure to recognize the need to promptly evacuate an area when exposed.

Dermal

Dermal absorption can occur, leading to systemic toxicity. Absorption occurs more readily at high ambient temperature and relative humidity (ATSDR 2006).

Because of their relatively larger surface area:body weight ratio, children are more vulnerable to toxicants absorbed through the skin.

Ocular/Ophthalmic

When splashed in the eye, hydrogen cyanide can cause eye irritation and swelling. Eye contact with cyanide salts has produced systemic symptoms in experimental animals (ATSDR 2006). Dilated pupils are common in severe poisoning. Transient blindness can occur, although it rarely does (HSDB, 2014).

Chronic Exposure

Chronic exposure may be more serious for children because of their potential longer life span.

Carcinogenicity

Hydrogen cyanide has not been classified for carcinogenic effects (IARC 2007; IRIS 2007; NTP 2005), and no carcinogenic effects have been reported for hydrogen cyanide.

Reproductive and Developmental Effects

No hydrogen cyanide-induced developmental effects have been reported in humans or in animals at exposure levels that were not maternally toxic (ATSDR 2006). Mild reproductive effects of sodium cyanide have been reported in rats and mice administered the substance orally for 13 weeks (ATSDR 2006). Increased levels of thiocyanate in the umbilical cords of fetuses whose mothers smoked compared to those whose mothers were non-smokers suggests that thiocyanate, and possibly also cyanide, can cross the placenta. No data were located pertaining to hydrogen cyanide in breast milk (ATSDR 2006).

Prehospital Management

A victim exposed to hydrogen cyanide gas could secondarily contaminate a rescuer attempting resuscitation without a respiratory barrier. Victims whose clothing or skin is contaminated with hydrogen cyanide liquid or solution can secondarily contaminate response personnel by direct contact or through off-gassing vapor. Avoid dermal contact with cyanide-contaminated victims or with gastric contents of victims who may have ingested cyanide-containing materials.

Hydrogen cyanide poisoning is marked by abrupt onset of profound toxic effects that may include syncope, seizures, coma, gasping respirations, and cardiovascular collapse, causing death within minutes. These effects can occur from all routes of exposure.

Hot Zone

Rescuers should be trained and appropriately attired before entering the Hot Zone. If the proper equipment is not available, or if rescuers have not been trained in its use, assistance should be obtained from a local or regional HAZMAT team or other properly equipped response organization.

Rescuer Protection

Hydrogen cyanide is a highly toxic systemic poison that is absorbed well by inhalation and through the skin.

Skin Protection: Chemical-protective clothing against hydrogen cyanide is recommended because both hydrogen cyanide vapor and liquid can be absorbed through the skin to produce systemic toxicity.

ABC Reminders

Quickly establish a patent airway, ensure adequate respiration and pulse. If trauma is suspected, maintain cervical immobilization manually and apply a cervical collar and a backboard when feasible.

Victim Removal

If victims can walk, lead them out of the Hot Zone to the Decontamination Zone. Victims who are unable to walk may be removed on backboards or gurneys; if these are not available, carefully carry or drag victims to safety.

Victims with chemically-induced acute disorders may suffer from anxiety, especially children who may be separated from a parent or other adult.

Decontamination Zone

Patients exposed only to hydrogen cyanide gas who have no eye irritation do not need decontamination. They may be transferred immediately to the Support Zone. Other patients will require decontamination as described below.

Rescuer Protection

If exposure levels are determined to be safe, decontamination may be conducted by personnel wearing a lower level of protection than that worn in the Hot Zone (described above). However, do not attempt resuscitation without a respiratory barrier.

ABC Reminders

Quickly establish a patent airway, ensure adequate respiration and pulse. Stabilize the cervical spine with a collar and a backboard if trauma is suspected. Administer supplemental oxygen as required. Assist ventilation with a bag-valve-mask device if necessary.

Basic Decontamination

Speed is critical. For symptomatic victims, provide treatment with 100% oxygen and specific antidotes as needed. Treatment should be given simultaneously with decontamination procedures. (For treatment, see ABC Reminders, Advanced Treatment, and Antidotes below).

Victims who are able may assist with their own decontamination. Quickly remove and double-bag contaminated clothing and personal belongings.

Flush exposed skin and hair with copious amounts of water for at least 20 minutes. Wash with soap and rinse thoroughly with water. Use caution to avoid hypothermia when decontaminating victims, particularly children or the elderly. Use blankets or warmers as needed.

Flush exposed or irritated eyes with plain water or saline for 5 minutes. Remove contact lenses if easily removable without additional trauma to the eye. Continue eye irrigation during other basic care or transport (HSDB 2007). If pain or injury is evident, continue irrigation while transferring to the Support Zone.

In cases of ingestion, do not induce emesis. If the victim is symptomatic, immediately institute emergency life support measures including the use of a cyanide antidote (see Antidotes below).

Provide reassurance to chemically-contaminated victims during decontamination, particularly children who may suffer separation anxiety if separation from a parent occurs.

Transfer to Support Zone

As soon as basic decontamination is complete, move the victim to the Support Zone.

Support Zone

Be certain that victims have been decontaminated properly (see Decontamination Zone above). Victims who have undergone decontamination pose no serious risks of secondary contamination to rescuers. In such cases, Support Zone personnel require no specialized protective gear.

ABC Reminders

Quickly establish a patent airway, ensure adequate respiration and pulse. If trauma is suspected, maintain cervical immobilization manually and apply a cervical collar and a backboard when feasible. Administer supplemental oxygen as required and establish intravenous access if necessary. Place on a cardiac monitor.

Patients who rapidly regain consciousness and who have no other signs or symptoms may not require antidotal treatment. Those who remain comatose or develop shock should be treated promptly with the antidotes in the cyanide antidote kit (see Antidotes below).

Additional Decontamination

Continue irrigating exposed skin and eyes, as appropriate.

In cases of ingestion, do not induce emesis. If the patient is symptomatic, immediately institute emergency life support measures, including the use of a cyanide antidote (see Antidotes below).

Advanced Treatment

In cases of respiratory compromise, secure airway and support respiration according to advanced life support (ALS) protocols.

Patients who are in shock or have seizures should be treated according to ALS protocols. These patients or those who have arrhythmias may be seriously acidotic; consider giving, under medical supervision, 1 mEq/kg intravenous sodium bicarbonate (HSDB 2007).

Antidotes

When possible, treatment with cyanide antidotes should be given under medical supervision to unconscious victims who have known or strongly suspected cyanide poisoning. There are currently two cyanide antidotal kits approved by the U.S. Food and Drug Administration (FDA). Use either the standard cyanide antidotal kit that includes amyl nitrite perles and intravenous infusions of sodium nitrite and sodium thiosulfate or the recently approved cyanokit that employs intravenous infusion of hydroxocobalamin (FDA 2006; HSDB 2007).

Use of the standard cyanide antidotal kit includes the following: Amyl nitrite perles should be broken onto a gauze pad and held under the nose, over the Ambu-valve intake, or placed under the lip of the face mask. Inhale for 30 seconds every minute and use a new perle every 3 minutes if sodium nitrite infusions will be delayed (HSDB 2007). If the patient has not responded to oxygen and amyl nitrite treatment, infuse sodium nitrite intravenously as soon as possible. The usual adult dose is 10 mL of a 3% solution (300 mg) infused over absolutely no less than 5 minutes; the average pediatric dose is 0.12-0.33 mL/kg body weight up to 10 mL infused as above. Monitor blood pressure during sodium nitrite administration, and slow the rate of infusion if hypotension develops (HSDB 2007). Next, infuse sodium thiosulfate intravenously. The usual adult dose is 50 mL of a 25% solution (12.5 g) infused over 10-20 minutes; the average pediatric dose is 1.65 mL/kg of a 25% solution. Repeat one-half of the initial dose 30 minutes later if there is an inadequate clinical response (HSDB 2007).

Use of the Cyanokit includes the following: Infuse hydroxocobalamin (5 g) intravenously over 15 minutes. Depending on the severity of poisoning and the clinical response, a second dose of 5 g may be administered over 15 minutes to 2 hours (based on patient condition). The recommended diluent is 0.9% sodium chloride. Some drugs are incompatible with hydroxocobalamin, in which case, a separate intravenous line may be necessary (Drugs.com 2007).

Transport to Medical Facility

Only decontaminated patients or patients not requiring decontamination should be transported to a medical facility. "Body bags" are not recommended.

Report to the base station and the receiving medical facility the condition of the patient, treatment given, and estimated time of arrival at the medical facility.

If a cyanide-containing solution has been ingested, prepare the ambulance in case the victim vomits. Have ready several towels and open plastic bags to quickly clean up and isolate vomitus.

Multi-Casualty Triage

Consult with the base station physician or the regional poison control center for advice regarding triage of multiple victims.

Patients with evidence of significant hydrogen cyanide exposure, and all patients with oral exposure to hydrogen cyanide and those with potentially hazardous dermal exposure, should be transported to a medical facility for evaluation.

Patients who have only brief inhalation exposure and mild or transient symptoms may be discharged from the scene after their names, addresses, and telephone numbers are recorded. They should be advised to seek medical care promptly if symptoms develop or recur (see Patient Information Sheet below).

Emergency Department Management

Hospital personnel in an enclosed area can be secondarily contaminated by vapor off-gassing from heavily soaked clothing or skin, or from vomitus. Avoid dermal contact with cyanide-contaminated patients or with gastric contents of patients who may have ingested cyanide-containing materials. Patients do not pose secondary contamination risks after contaminated clothing is removed and the skin is washed.

Hydrogen cyanide poisoning is marked by abrupt onset of profound health effects that may include syncope, seizures, coma, gasping respirations, and cardiovascular collapse, causing death within minutes.

Patients exposed to hydrogen cyanide can survive with supportive care and rapid administration of specific antidotes.

Decontamination Area

Previously decontaminated patients and patients exposed only to hydrogen cyanide gas and have no skin or eye irritation may be transferred immediately to the Critical Care Area. Other patients require decontamination as described below.

ED personnel should don butyl rubber gloves and aprons before treating patients who have been exposed to hydrogen cyanide liquid or solutions. Hydrogen cyanide readily penetrates most rubbers and barrier fabrics or creams, but butyl rubber provides good skin protection for a short period of time.

Be aware that use of protective equipment by the provider may cause anxiety, particularly in children, resulting in decreased compliance with further management efforts.

Because of their relatively larger surface area:body weight ratio, children are more vulnerable to toxicants absorbed through the skin.

ABC Reminders

Evaluate and support airway, breathing, and circulation according to ALS protocols.

Patients who are comatose, hypotensive, or have seizures or cardiac dysrhythmias should be treated in the conventional manner. If not previously administered, give sodium bicarbonate intravenously to these patients (HSDB 2007). Further bicarbonate therapy should be guided by ABG measurements.

Basic Decontamination

Patients who are able may assist with their own decontamination. Remove and double-bag contaminated clothing and personal belongings.

Speed is critical. If the patient is symptomatic, immediately institute emergency life support measures, including the use of a cyanide antidote kit (see Antidotes in the Prehospital Management section above).

If the patient's clothing is wet with hydrogen cyanide solution, quickly remove contaminated clothing while flushing exposed skin and hair with plain water for 2-3 minutes (preferably under a shower), then wash twice with mild soap (HSDB 2007). Use caution to avoid hypothermia when decontaminating victims, particularly children or the elderly. Use blankets or warmers as needed.

Irrigate exposed eyes for at least 5 minutes. Remove contact lenses if easily removable without additional trauma to the eye. Continue irrigation while transporting the patient to the Critical Care Area.

In cases of ingestion, do not induce emesis. If the victim is alert, asymptomatic, and has a gag reflex, consider administering a slurry of activated charcoal at a dose of 1 g/kg (infant, child, and adult dose) (HSDB 2007). Because cyanide absorption from the gut is rapid, the effectiveness of activated charcoal will depend on how quickly after ingestion it can be administered. Consider gastric lavage if the patient is conscious and it can be performed shortly after ingestion. Isolate gastric washings and vomitus; they may off-gas hydrogen cyanide (HSDB 2007).

Critical Care Area

Be certain that appropriate decontamination has been carried out (see Decontamination Area above).

ABC Reminders

Evaluate and support airway, breathing, and circulation as in ABC Reminders above under Decontamination Zone. Establish intravenous access in seriously ill patients if this has not been done previously. Continuously monitor cardiac rhythm.

Patients who are in shock or have seizures should be treated according to ALS protocols. These patients or those who have dysrhythmias may be seriously acidotic; consider giving 1 mEq/kg intravenous sodium bicarbonate (HSDB 2007).

Inhalation Exposure

Inhalation is the primary route of exposure to hydrogen cyanide. Refer to Antidotes and Other Treatments below for appropriate clinical treatment of systemic effects.

Skin Exposure

If the skin contacted hydrogen cyanide liquid or cyanide solutions, chemical burns may occur; treat as thermal burns. Watch for signs or symptoms of systemic toxicity, which may be delayed in onset for up to 1 hour.

Eye Exposure

Continue irrigation for at least 15 minutes. Test visual acuity. Examine the eyes for corneal damage and treat appropriately. Immediately consult an ophthalmologist for patients who have corneal injuries.

Ingestion Exposure

Do not induce emesis. If the victim is symptomatic, immediately institute emergency life support measures including the use of a cyanide antidote kit (see Antidotes in the Prehospital Management section above). If the victim is alert, asymptomatic, has a gag reflex, and it has not been done previously, consider performing gastric lavage.

Vomitus or gastric washings should be isolated (e.g., by attaching the lavage tube to isolated wall suction or another closed container).

Antidotes and Other Treatments

Patients who have signs or symptoms of significant systemic toxicity should be evaluated for antidotal treatment. There are currently two cyanide antidotal kits approved by the U.S. FDA. Use either the cyanide antidotal kit that includes amyl nitrite perles and intravenous infusions of sodium nitrite and sodium thiosulfate or the recently approved cyanokit that employs intravenous infusion of hydroxocobalamin (FDA 2006; HSDB 2007). For instructions on use of cyanide antidotal kits, see Antidotes in the Prehospital Management section above.

The efficacy of hyperbaric oxygen in cyanide poisoning is unproven. It has been reported to be useful in severe cases of smoke inhalation combined with exposure to hydrogen cyanide and carbon monoxide (HSDB 2007).

Laboratory Tests

The diagnosis of acute cyanide toxicity is primarily a clinical one (based on rapid onset of CNS toxicity and cardiorespiratory collapse). Specific tests for the presence of cyanide in blood and urine may be useful in confirming exposure, but have limited usefulness in acute treatment decisions. Routine laboratory studies for all exposed patients include CBC, blood glucose, and electrolyte determinations. Additional studies for patients exposed to hydrogen cyanide include ECG monitoring, determinations of serum lactate and urinary thiocyanate, chest radiography, and pulse oximetry (or ABG measurements) (Hall and Rumack 1998). Some pulse oximeters may give spurious results in the presence of hemoglobin species other than oxyhemoglobin and deoxyhemoglobin.

After treatment with nitrites, serum methemoglobin levels may be monitored. Whole blood cyanide tests generally require several hours and cannot be used to guide emergency treatment. However, blood cyanide levels may be useful in documenting exposure (Hall and Rumack 1998; Kruszyna et al. 1993).

MRI studies may be useful in identifying the location and extent of brain injury in patients with cyanide-induced Parkinsonian syndrome.

Disposition and Follow-up

Consider hospitalizing patients who have histories of significant exposure and are symptomatic. Whenever infusions from a cyanide antidote kit are used, the patient should be admitted to the intensive care unit (Hall and Rumack 1998).

Delayed Effects

Patients who have ingested hydrogen cyanide solutions or patients who have direct skin or eye contact should be observed in the Emergency Department for at least 4 to 6 hours (Hall and Rumack 1998).

Patient Release

Patients who remain asymptomatic 4 to 6 hours after exposure may be discharged with instructions to seek medical care promptly if symptoms develop (see the Hydrogen Cyanideâ€”Patient Information Sheet below).

Follow-up

Obtain the name of the patient's primary care physician so that the hospital can send a copy of the ED visit to the patient's doctor.

Survivors of a serious exposure should be evaluated for ischemic damage to the brain and heart. Patients who have serious systemic cyanide poisoning may be at risk for CNS sequelae including Parkinsonian-like syndromes; they should be monitored for several weeks to months (Hall and Rumack 1998).

Patients who have corneal injuries should be reexamined within 24 hours.

Reporting

If a work-related incident has occurred, you may be legally required to file a report; contact your state or local health department.

Other persons may still be at risk in the setting where this incident occurred. If the incident occurred in the workplace, discussing it with company personnel may prevent future incidents. If a public health risk exists, notify your state or local health department or other public agency. When appropriate, inform patients that they may request an evaluation of their workplace from OSHA or NIOSH. See Appendices III and IV for a list of agencies that may be of assistance.

Patient Information Sheet

This handout provides information and follow-up instructions for persons who have been exposed to hydrogen cyanide.

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What is hydrogen cyanide?

At room temperature, hydrogen cyanide is a volatile, colorless-to-blue liquid (also called hydrocyanic acid). It rapidly becomes a gas that can produce death in minutes if breathed. Hydrogen cyanide is used in making fibers, plastics, dyes, pesticides, and other chemicals, and as a fumigant to kill rats. It is also used in electroplating metals and in developing photographic film. Low cyanide levels can be measured in cigarette smoke.

What immediate health effects can be caused by exposure to hydrogen cyanide?

Breathing small amounts of hydrogen cyanide may cause headache, dizziness, weakness, nausea, and vomiting. Larger amounts may cause gasping, irregular heartbeats, seizures, fainting, and even rapid death. Generally, the more serious the exposure, the more severe the symptoms. Similar symptoms may be produced when solutions of hydrogen cyanide are ingested or come in contact with the skin.

Can hydrogen cyanide poisoning be treated?

The treatment for cyanide poisoning includes breathing pure oxygen, and in the case of serious symptoms, treatment with specific cyanide antidotes. Persons with serious symptoms will need to be hospitalized.

Are any future health effects likely to occur?

A single small exposure from which a person recovers quickly is not likely to cause delayed or long-term effects. After a serious exposure, a patient may have brain or heart damage.

What tests can be done if a person has been exposed to hydrogen cyanide?

Specific tests for the presence of cyanide in blood and urine may be useful in confirming exposure, but have limited usefulness in acute treatment decisions. If a severe exposure has occurred, blood and urine analyses and other tests may show whether the brain or heart has been injured. Testing is not needed in every case.

Where can more information about hydrogen cyanide be found?

More information about hydrogen cyanide can be obtained from your regional poison control center; your state, county, or local health department; the Agency for Toxic Substances and Disease Registry (ATSDR); your doctor; or a clinic in your area that specializes in occupational and environmental health. If the exposure happened at work, you may wish to discuss it with your employer, the Occupational Safety and Health Administration (OSHA), or the National Institute for Occupational Safety and Health (NIOSH). Ask the person who gave you this form for help in locating these telephone numbers.

ATSDR can also tell you the location of occupational and environmental health clinics. These clinics specialize in
recognizing, evaluating, and treating illnesses resulting from exposure to hazardous substances.